Polyhydric alcohols or polyols possess considerable economic significance as a condensation component for forming polyesters or polyurethanes, synthetic resin coatings, lubricants and plasticizers. In this context, polyhydric alcohols of interest are particularly those which are obtained by a mixed aldol addition of formaldehyde with iso- or n-butyraldehyde. The aldol addition between formaldehyde and the appropriate butyraldehyde first forms an aldehydic intermediate which then has to be reduced to the polyhydric alcohol. An industrially important polyhydric alcohol obtainable by this method is neopentyl glycol [NPG, 2,2-dimethylpropane-1,3-diol] formed from the mixed aldolization of formaldehyde and isobutyraldehyde.
The aldol addition is carried out in the presence of basic catalysts, for example, alkali metal hydroxides or aliphatic amines, and initially affords the isolable hydroxypivalaldehyde (HPA) intermediate. This intermediate can subsequently be converted with excess formaldehyde in accordance with the Cannizzaro reaction to neopentyl glycol to form one equivalent of a formate salt. In this configuration of the reduction step, the formate salt is therefore obtained as co-product and the cost-effectiveness of this method variant also depends on the commercial opportunities for the formate salt. However, also implemented industrially is the catalytic hydrogenation of hydroxypivalaldehyde in the gas and liquid phase over a metal catalyst. Suitable hydrogenation catalysts have been found, according to EP 0 278 106 A1, to be nickel catalysts. Catalysts based on copper, zinc and zirconium are used in the hydrogenation step in the method according to EP 0 484 800 A2.
Copper chromite catalysts are also frequently used for the hydrogenation of hydroxypivalaldehyde. Copper chromite catalysts frequently comprise other metals as activators, for example barium, cadmium, magnesium, manganese and/or a rare earth metal. According to U.S. Pat. No. 4,855,515, manganese-doped copper chromite catalysts in particular excel in the hydrogenation of the aldolization product of the reaction of formaldehyde with isobutyraldehyde. WO98/29374 A1 discloses the use of a barium-doped copper chromite catalyst for the hydrogenation of hydroxypivalaldehyde in a methanolic solution.
According to the teaching of DE 1 518 784 A1, a mixture of hydroxypivalaldehyde and excess isobutyraldehyde is hydrogenated to neopentyl glycol and isobutanol in the presence of a copper chromite catalyst which has been doped with barium. According to EP 0 006 460 A1, the two-step high pressure hydrogenation of crude hydroxypivalaldehyde, which is carried out with increasing hydrogenation temperatures, also uses a copper chromite catalyst activated with barium.
GB 2 482 887 A discloses the use of copper chromite catalysts, doped with both manganese and barium, for the hydrogenation of furfural. Distribution of product can be steered in the direction of furfuryl alcohol or 2-methylfuran, according to the choice of hydrogenation temperatures.
EP 0 301 853 A1 describes copper chromite catalysts which may include both barium and manganese as further adjuvants. The known catalysts can be used for the hydrogenation of aldehydes.
The copper chromite catalysts known from the prior art operate at comparatively high hydrogenation temperatures.
Copper chromite catalysts not only possess a good hydrogenation activity regarding conversion of hydroxypivalaldehyde to neopentyl glycol, but they are also sufficiently active so as to cleave by-products, which are formed during aldolization of isobutyraldehyde with formaldehyde, under hydrogenating conditions and to liberate neopentyl glycol bound to the by-products. Such by-products are, for example, neopentyl glycol isobutyrate or the Tishchenko ester, neopentyl glycol monohydroxypivalate, formed by the disproportionation of hydroxypivalaldehyde. The specific cleavage by hydrogenation of high-boiling by-products from the neopentyl glycol preparation using copper chromite catalysts is described in DE 10 2008 033163 A1.
EP 0 522 368 A1 discloses carrying out the hydrogenation of hydroxypivalaldehyde in a solution comprising at least 20% by weight of a low molecular weight alcohol, for example methanol or n-butanol, based on the mixture of alcohol and reaction product, and also water in an amount of not more than 40% by weight, based on the total amount of water, alcohol and reaction product. The hydrogenation catalyst recommended is a copper chromite catalyst.
As a product produced industrially, neopentyl glycol has a major economic significance and thus there always exists a need to improve the known methods for preparing neopentyl glycol, whether by improving the product yield, by better utilization of plant equipment or by a lowering of energy input.